Artifacts in a CRW file?

PatrickB · #1 09-21-2005, 04:29 PM

Hi all,

today I got a CD with a bunch of photos in CRW format, but they are just around 4MB each. I was surprised but converted them to a TIFF and found JPEG artifacts?

Is this my fault or can things like this happen?

Patrick

Kraellin · #2 09-21-2005, 06:32 PM

hi patrick,

i'm not familiar with the crw format, so cant say for sure. normally, most conversions are fairly simple and straight-forward, leaving generally a pretty good duplication, at least in my experience. so, my guess, and it's a straight guess, is that wherever you got the cd from, someone on that end did something. they may have converted to tiff from jpg or done something else.

it's also true that the 'jpg artifact' thing doesnt always come from jpg's. is it possible someone did some work on them before sending them to you? and, if these came from a digital camera where it saves in .jpg, they might have been converted later to something else.

you might also want to try a different converter just as a cross check to make sure it's not your converter that's causing the problem.

but generally, conversions lose very little data, if any. the main difference in formats is usually in the header information.

Craig

Mike · #3 09-21-2005, 10:08 PM

I believe that CRW is the Canon RAW format, what are you using to convert them? And do you know what camera they are out of?

v.bampton · #4 09-22-2005, 12:23 AM

You're quite right about CRW being RAW files from a Canon, therefore you can pretty much rest assured they are directly off the camera without ever having been a jpg. There's often a THM file as well, which holds a thumbnail and sometimes some extra exif header data.

Looking at the exif data is likely to be the best way of finding out what's going on. Could it be noise issues you're seeing? Alternatively, the software you're using to convert could be the issue.

Can you upload one anywhere, and we'll take a look? If not, email it to me (my email is in my profile).

EDIT - ok, email address is not in my profile! It's photoshop AT ntlworld DOT com.

Victoria

Caitlin · #5 09-22-2005, 03:34 AM

There is something distincly fishy with those files I think - can't imaging a RAW file could come in at only 4mb.

v.bampton · #6 09-22-2005, 06:50 AM

A Canon 10d comes in at between about 4.5mb and 7.5mb depending on the picture. A 20d is 6.5mb and 10mb.

I'm guessing there must be a consumer level Canon that will shoot some RAW (maybe 5mp?), or it could be a 10d/350d.

We're all playing guessing games until we see the file though.

Doug Nelson · #7 09-22-2005, 12:15 PM

Evidently, Canon (and some other manufacturers) actually does let you choose a level of JPG compression for Raw files. This seems totally bonkers to me, but I've been assured by someone that is in a position to know that JPG artifacting within a Canon Raw file is a very real possibility. It can, however, be avoided in the future by changing settings in the camera.

v.bampton · #8 09-22-2005, 03:43 PM

That's interesting Doug. I hadn't come across that. I'd assumed that the file size varied depending simply on the detail in the picture (we shoot weddings and portraits, and the file sizes vary across the day, without changing any camera settings). Any links to further info? Thanks!

Doug Nelson · #9 09-22-2005, 08:54 PM

That's one clue, right there. If filesize changes due to content, compression is happening. Uncompressed files of the same pixel dimensions will be the same size regardless of content.

It turns out to be even more complex than I heard. CRW consists of both a JPG and a RAW file simultaneously, and various programs can have access to both, or only the JPG. So that's good news, the Raw is still RAW. But it sounds like in this case only the embedded JPG was readable.

v.bampton · #10 09-23-2005, 03:30 AM

That makes sense. In that case, any idea which camera setting affects the compression, because we'd rather have it turned off ourselves!

I guess I'm off to Google that one then! Thanks for the tip off!

PatrickB · #11 09-23-2005, 04:24 AM

Doug, If I didn't get you wrong you meant my conversion program took the info from the jpeg rather than the real ras information? But in a 4MB raw for a quite big image (usual 2000 pixels x yada around like that) how tiny would the jpeg be? I'll try to put one online later one!

Patrick

v.bampton · #12 09-24-2005, 03:52 AM

Ok, I had to google this one, as image quality is paramount in our end of the market, and I was concerned that maybe we weren't getting the best from our cameras.

So, for anyone else who's interested, here's what I've come up with...

Canon DO compress their RAW files - but it's not a jpeg compression, it's a proprietary lossless algorithm, so there are not artifacts caused by this compression. (Confirmation of this on Canon's website http://consumer.usa.canon.com/ir/con...=8772&pageno=2) The only jpeg involved is the tiny thumbnail.

RAW + Jpg is a different setting entirely - and gives 2 files - a normal jpeg, and a normal CRW.

There is nothing you can change in camera to affect the compression on a CRW file.

Hope that helps to clarify it for anyone who's interested. It certainly put my mind at rest.